Hello, 8800 RPM

So how’s it drive? Twist the ignition key and you’re greeted by the GT3’s characteristic instant-on spin-up and easy revs, but there’s a slightly gruffer bark from the twin tail­pipes. Let the clutch out and the difference in power is anything but subtle. As the 3.9 pulls through second gear and then third, any initial skepticism about the claimed 500 horsepower fades.

This is a ridiculously fast 911. We’ve driven faster turbocharged 911s, but this is the fastest normally-aspirated street 911 we’ve tested. By a mile. It’s Turbo fast without Turbo torque. Instead, big speed comes with big revs. As a result, it feels manic. May­be too manic, because you feel like you’re getting away with something whenever the tach hits 8800 rpm. Whether it’s a rod, wrist pin, or your driver’s license, you’re sure something’s got to give. Then you do it again. And again…

That’s not the 3.9’s best party trick, however. No, the shocker is this: It’s actually smoother than the stock 3.6, and not just a little. Where the factory 3.6 is grainy — some go so far as to call it rough-running — the 3.9 isn’t. Ross says that a few tricks help here, the most surprising among them being different spark plugs.

Ten miles in, I’m hooked. The soundtrack is incredible, with a demonic shriek underpinned by lower frequencies normally associated with air-cooled 911s. But it’s the way that the 3.9 pulls smoothly from 3000 rpm to 8800 rpm that’s intoxicating. Second gear is good for more than 90 mph while third will take you to the far side of 120 — and there are three more gears to go. In practice, there’s no need to shift out of second on a back road anymore, which gets us thinking about lower ratios in second through fifth to multiply the torque. The grip to use it is there. Then again, this thing is fast enough.

Too bad the chassis isn’t quite as magical. While the alignment helps, the RS’s first-gen PASM dampers still disappoint. The 2010 GT3’s chassis is noticeably better, but we barely remember it because the 3.9 is a scene stealer. When we get back in the red GT3, it feels like someone has hit the Mute button and muted more than noise. The 3.8, while fantastic, can’t hold a candle to the 3.9. Yes, it’s that big of a difference. The 3.9 feels far more alive, far more willing, and far more thrilling.

Does it feel like 500 horsepower? Yes. Does it spin like a GT3 RSR? It does. Will it blow up? We can’t say, but the car will go on to run over a thousand miles in our care on 91-octane gas without a single hiccup. Particularly impressive is the EVOMSit software tuning. Partial-throttle response, often a weak spot with aftermarket tunes, is every bit as good as a factory GT3’s. It’s the final piece in that rarest of puzzles: an aftermarket engine that outshines its factory basis.

There are caveats, of course. We can’t speak to longterm reliability, and such a conversion voids any re­maining warranty coverage on the powertrain. Then there’s the not-so-small matter of the 13:1 compression ratio. That said, the engine management seemed able to compensate, as the 3.9 never pinged in our time with it. Finally, there’s this: While the engine looks stock, emissions legality is another matter — though Ross says it passes the sniffer test with flying colors.

All serious considerations, but they’re not what’s on my mind weeks after the RS leaves. What’s on my mind is the winner of this matchup, and it’s not a car. It’s an engine, and I’m a handling guy. As that sinks in, I realize something else.

Much as I love short-stroke 2.5s and 2.8s, today’s GT3 engines eclipse them. But this 3.9 vanquishes every GT3 flat six I’ve tried. Including the ones in the 997 GT3 Cup and RSR? In a word, yes —in part because they’re overshadowed by a sequential transmission that makes F1 noises and in part because they’re not built to be enjoyable — they’re built to be raced. The 3.9 feels RSR strong but more refined than the production 2010 GT3’s 3.8. Put simply, it’s the best normally-aspirated flat six I’ve tried.

That ranks it up there with my top two Porsche engines of all time: the 469+hp, twin-turbo flat six in 1988’s Ruf CTR and the 605-hp, 5.7-liter V10 in 2004’s Carrera GT. It’s hard to see how a flat six can get any better, but perhaps Porsche under Ferdi­nand Piëch will show us.